Thermoplastic polyurethane (TPU) elastomers are composed of easily movable long-chain units called soft segments and extremely crystalline units called hard segments and exhibit excellent physical properties ascribed to their characteristic structure, such as elasticity, elongation, mechanical strength, and abrasion resistance. With these physical properties taken into advantage, they have been used in a variety of fields as, for example, hoses, belts, wires, cables, pipes, soles, automotive interior and exterior trim, and various moldings.
A flame retardant is blended with a TPU to impart flame retardancy to the TPU. The use of a halogen flame retardant has the problem of harmful gas generation on combustion. It is known to use instead a nitrogen- and/or phosphorus-containing compound, such as melamine phosphate, melamine polyphosphate, or a condensed phosphoric ester, as a halogen-free flame retardant (see patent document 1 below).
It is also proposed to use an intumescent phosphate flame retardant that induces formation of a swollen surface layer on combustion, which prevents diffusion of a decomposition product and heat transfer thereby to achieve flame retardation (see patent document 2 below).
However, the conventional phosphate-based flame retardants should be added in large quantities so as to obtain sufficient flame retardation, which can result in impairment of the physical properties intrinsic to the resin. The use of an anti-drip agent, such as polytetrafluoroethylene, which is added to prevent dripping, can also leads to the impairment of the physical properties intrinsic to the resin.